Is it permitted to use a postage stamp with an image of the idol god Baal whom the Prophet Elias (Peace and blessing be upon him) was against in the Holy Qur'an (37:125-126)?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

A Muslim should not voluntarily use the postal stamps with pictures of living beings or idols of Kuffars or stamps containing their religious slogans. If one does not have any other choice other than to use such stamps he may use them but only in the limits of necessity.
As for the Ba'l mentioned in verse "And verily, Iliyâs (Elias) was one of the Messengers. When he said to his people: "Will you not fear Allâh? "Will you call upon Ba'l and forsake the Best of creators, "Allâh, your Lord and the Lord of your forefathers?" ". [37:123-126]
The Mufassirs (Interpreters of Qur'an) say that the word "Ba'l" here in the above Verse means lord worshipped by Kuffars. Some others believe that it was a name of a woman worshipped by the nation of Ilyal (peace be upon him).
Some other Mufassirs believe that Ba'l was a name of an idol who used to be worshipped by the people of Ba'l bak (a city in Lebanon) and they say that the city is named after his name.
Anyway, we do not know any scholar who has confirmed that the god Ba'l mentioned in your question is the same Ba'l which is mentioned in the Verse. But the possibility of being the same ones could not be rejected.
Allah knows best.